1. Field of the Invention
The present invention relates generally to a fuel cell, and more particularly to a fuel cell using a polymer electrolyte membrane.
2. Description of the Related Art
A fuel cell is an electrochemical energy conversion device. Fuel cells use an electrolyte membrane to catalytically react an input fuel, such as hydrogen and oxygen, to product an electrical current. The electrolyte material is sandwiched between two electrodes (an anode and a cathode). The input fuel passes over the anode (and oxygen over the cathode) where the fuel catalytically splits into ions and electrons. The electrons go through an external circuit that serves as an electric load while the ions move through the electrolyte toward the oppositely charged electrode. At the second electrode, the ions combine to create by-products of the energy conversion process, the byproducts being primarily water and heat. There are several types of fuel cells and the type is based on the electrolyte employed, including: a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, and a polymer electrolyte membrane fuel cell, also referred to as a proton exchange membrane fuel cell.
The type of fuel cell that involves a polymer electrolyte membrane is hereinafter referred to as a PEM fuel cell. Work on PEM type fuel cells has produced fuel cells in the size range of only 0.2 millimeters in thickness and capable of running for over 60,000 hours at 80 degrees Celsius. These kinds of PEM fuel cells are capable of producing better than 400 mA (milliamperes) of current per square centimeter, at 0.7 volts, in some applications, and depending on whether air or oxygen is used on the cathode. Stacking of the cells is required to deliver higher voltages.
While progress in this technology area has been excellent and remarkable gains have been made in miniaturization, a major challenge confronting those working in this field is what is termed in the industry as mass transport, or the management of the internal water movement and water byproduct of the fuel cell process. This problem of water byproduct elimination is further complicated by the fact that water is also required as part of the process of operation and for cooling the fuel cells. Cooling is very important in fuel cell operation because as much as one third of the available energy is released as heat in the stack and must be removed. If the water is not removed or is not removed quickly enough, the fuel cell will flood out and stop generating electricity. The steep drop in performance (the drop in voltage) as shown in the chart in FIG. 1 as a change in the output from 0.8 mA cm2 to 1.4 mA cm2 and labeled mass transport is the result of water build up in the fuel cell stack. It would be desirable to at least reduce this loss in power.
Fuel cell expert Gregor Hoogers describes the critical nature of the problem of managing fuel cell water byproduct in the publication Fuel Cell Technology Handbook (CRC Press; ISBN 0-8493-0877-1). In Chapter 4, entitled “Fuel Cell Components and their Impact on Performance” Hoogers describes the water byproduct problem several times, particularly in paragraphs 4.2, 4.2.4, 4.3.1.2 and 4.4.1.1 where the issue and problems relating to water flow are discussed in detail. It is clear that water and its control is a major factor limiting the use of PEM fuel cells especially at high current draws per unit area and, perhaps, in other fuel cell technologies as well.
In applications where a PEM fuel cell is designed to be operational and to supply power for only a finite amount of time (as for example in the case of ordnance applications where the fuel cell is needed for mere minutes at most) the issue of water control may not be critical. But in applications requiring operational periods of substantially longer time periods such in the use of PEM cells in autos and in stationary power generation applications, the accumulation of the water byproduct causes major problems in terms of fuel cell operation, durability and efficiency.